Electrostatic pattern-coupled digitizer

ABSTRACT

A digitizer in which position of a free moving cursor or stylus over a digitizing surface is determined by electrostatic coupling of the cursor with three coplanar electrodes imbedded in the digitizing tablet. The electrodes consist of a repetitive sawtooth pattern to determine position in one axis, the sawtooths interleaved with progressively wider, repetitive stripes to determine position in another axis. The third electrode covers the remainder of the coplanar surface area. Only three electrode signals are required to be electronically processed to determine absolute vertical and horizontal position of the cursor over the digitizing tablet by a ratiometric measurement. The signals from the electrodes are processed synchronously to reject noise.

BACKGROUND OF INVENTION

This application is a continuation of my application Ser. No. 704,329;filed Feb. 21, 1985, now U.S. Pat. No. 4,705,919.

This invention relates to a digitizer in which electrostatic couplingbetween patterned electrodes of a digitizer tablet and a cursor orstylus movable thereon creates signals indicative of the position of thecursor on the tablet.

Devices which are the general subject of this invention are commonlyreferred to as "digitizers." The earliest digitizers were mechanicaldevices with restrained arms. Later digitizers have free-moving cursors,and use various forms of coupling phenomenon between a cursor and adigitizing tablet, such as electromagnetic, electrostatic, and evensound coupling. Signals in the tablet have been handled by means ofmagnetostrictive principles, resistive surfaces, or other conductors,but generally these devices depend upon monitoring of signals introducedin a plurality of X and Y conductors or a resistive film. Since signalsfrom each of the conductors need to be processed individually, orsignals from several points on the periphery of a resistive layer, thecomplexity and cost of this type of digitizer, including itselectronics, is high. Thus, there is a need for an improved digitizer.

SUMMARY OF THE INVENTION

A digitizer according to the invention herein comprises a digitizingtablet with a patterned electrode for each coordinate of atwo-coordinate system and a cursor with a coupling electrode movable onthe tablet, wherein electrical field coupling is utilized to ascertainthe position of the cursor on the digitizing tablet. The coordinatesystem is typically the X-Y coordinate system. The cursor comprises ahousing movable on the digitizing tablet, the housing containing acoupling electrode to which an input signal is applied. The couplingelectrode electrically field couples to sawtooth and stripe patternedelectrodes imbedded in the digitizing tablet. One of the sawtooth orstripe patterned electrodes is used to ascertain X-axis position of thecursor, and the other is used to ascertain Y-axis position of thecursor. The sawtooth and stripe patterned electrodes are convenientlyprovided coplanar on a printed circuit board and covered with a layer ofdielectric material to provide a smooth tablet surface. The signals onthe patterned electrodes are processed to ascertain the position of thecursor.

In accordance with more specific aspects of the invention, the digitizertablet has three coupling electrodes which produce an X-axis signal, aY-axis signal and a balance signal. The X-axis coupling electrodecomprises a plurality of conductive rectangular stripes deployedvertically and arranged in order of increasing width from one side tothe other side of the tablet, e.g., increasing in width from the left tothe right side of the tablet. The Y-axis coupling electrode comprises aplurality of conductive sawtooths also arranged vertically, with theY-axis sawtooths interleaved with the X-axis stripes. Movement of thecursor in the X-axis has a negligible effect on the coupling with theY-axis electrode and movement of the cursor in the Y-axis has negligibleeffect in coupling with the X-axis electrode. The balance electrodecovers the area of the digitizer tablet not covered by the X and Yelectrodes. A perimeter shield electrode surrounds the X, Y and balanceelectrodes, and a planar bottom shield electrode is provided below them,for shielding purposes.

The cursor electrode located in the cursor housing and movable on thesurface of the digitizing tablet surface is connected to an oscillatorwhose frequency typically is in the range of 10 KHZ to 1 MHZ. The cursorelectrode couples electrical signals into X-axis, Y-axis and balanceelectrodes. A shield electrode may be provided in the cursor over thecoupling electrode.

The surface area of X-axis electrode coupled with the cursor increasesas the cursor is moved or positioned toward the right side of thetablet, where the stripes are wider, and hence more electrical signal iscoupled to the X-axis electrode. Similarly, the surface area of theY-axis electrode coupled with the cursor increases as the cursor ismoved downwardly on the tablet, where the sawtooths are thicker, andhence more electrical signal is coupled to the Y-axis electrode. SinceX, Y and balance electrodes completely cover the area of the tabletexcept for small separation spacing, it therefore follows that the sumof the signals on the X-axis, Y-axis and balance electrodes will beindependent of cursor position. This sum signal depends primarily uponthe amplitude of the oscillator signal, the proximity of cursorelectrode to X-axis, Y-axis and balance electrodes and the dielectricconstant of the material between the cursor electrode and X-axis, Y-axisand balance electrodes.

The X-axis coordinate of the cursor is proportional to the X-axis signaldivided by the sum of X-axis, Y-axis and balance electrode signals.Similarly the Y-axis coordinate of the cursor is proportional to theY-axis electrode signal divided by the sum of X-axis, Y-axis and balanceelectrode signals. This ratiometric measurement technique measuresaccurately the cursor position and is independent of the proximity ofthe cursor to the digitizer tablet and the dielectric therebetween.

Since the digitizer operates at a single frequency, the electronicprocessing of the three electrode signals is accomplished by synchronousdetection, which results in excellent signal-to-noise ratio; even veryhigh levels of external noise are rejected. Also, since only threesignals are utilized, the electronics are relatively simple andinexpensive.

A reversal of the function of the cursor is within the scope of theinvention. The input signals may be sequentially applied to the X-axis,Y-axis and balance electrodes, and output signals synchronously detectedfrom the cursor.

The result is an accurate digitizer which can be fabricated at low cost.

OBJECTS OF THE INVENTION

Accordingly, it is a principal object of the invention herein to providean accurate digitizer.

An additional object of the invention is to provide a digitizer whichresists interference.

It is another main object of the invention herein to provide an accuratedigitizer of low complexity and cost.

Other and more specific features and objects of the invention will inpart be obvious and will in part appear from the following descriptionof the preferred embodiment and claims, taken together with thedrawings.

DRAWINGS

FIG. 1 is a perspective, partially schematic view of a digitizeraccording to the invention herein being used with a computer system;

FIG. 2 is a sectional view of the cursor and a portion of the tablet ofthe digitizer of FIG. 1, taken along the lines 2--2 of FIG. 1;

FIG. 3 is a sectional view of the tablet of the digitizer of FIG. 1,taken along the lines 3--3 of FIG. 1;

FIG. 4 is an enlarged segment of FIG. 3, showing detail of the tablet;

FIG. 5 is a schematic circuit diagram of the digitizer of FIG. 1; and

FIG. 6 is a graph showing operation of the circuit of FIG. 5.

The same reference numerals refer to the same elements throughout thevarious figures.

DESCRIPTION OF PREFERRED EMBODIMENT

Digitizers according to the invention herein generally comprise adigitizer tablet having separate patterned electrodes for eachcoordinate of a two-coordinate system, and a cursor movable on thesurface of the digitizer tablet and including a coupling electrode forestablishing electrical field coupling with the patterned electrodes ofthe digitizer tablet. A signal is applied to the coupling electrode andelectrostatically coupled to the patterned electrodes, and the resultingsignals on the patterned electrodes are utilized to calculate thecoordinates of the cursor position.

A digitizer 10, which is a preferred embodiment of the invention herein,is illustrated in FIGS. 1-6. The digitizer 10 generally comprises acursor 20, a tablet 40, and electronics 100 for providing an inputsignal to the cursor and processing output signals from the tablet.

As illustrated in FIG. 1, the digitizer 10 is generally used as an inputdevice to a computer 2. The computer 2 generally has a monitor 3associated therewith, and the monitor 3 may display numeric values ofthe coordinates representing the position of the cursor 20 on the tablet40, as indicated at 4 and 5 on the monitor display screen. The monitormay also be used to display a cursor dot, indicated at 6, the positionof which on the screen of the monitor is controlled by the position ofthe cursor on the tablet 40. Thus, the digitizer 10 may perform thefunction of cursor steering, if desired.

A cross-sectional view of the cursor 20 and a fragmentary portion of thetablet 40 is shown in FIG. 2. The cursor 20 generally comprises ahousing 21 having a depending peripheral side wall 22 which slides onthe surface of the digitizer tablet 40. The housing 21 mounts a couplingelectrode 30 substantially parallel to the digitizer tablet 40. Thecoupling electrode is fabricated of metal or other conductive material.The cursor 20 also mounts a ground or shield electrode 32, which isseparated from the coupling electrode 30 by a dielectric spacer 31. Theshield electrode 32 is preferably somewhat larger than the couplingelectrode 30, and overhangs the coupling electrode 30 about its marginaledge.

A cord 25 connects the cursor 20 to the digitizer electronics 100, thecord 25 comprising an input signal wire 26 connected to the couplingelectrode at 27 and a coaxial surrounding conductor 28 connected to theshield electrode 32 at 29. The signal is provided to the couplingelectrode 30 over the signal wire 26, as will be more fully discussedbelow.

The coupling electrode 30, dielectric spacer 31 and ground electrode 32may be provided in an annular shape and together with the cover 21define a central opening 34 in which crosshairs 35 are positioned (Seealso FIG. 5). The crosshairs assist in positioning the cursor over aspecific point, and the crosshairs are centered with respect to thecoupling electrode. In any event, the coupling electrode is preferablycircular, whereby orientation of the cursor does not affect couplingtherefrom.

As seen in FIG. 1, the cursor housing 21 may have a shape whichconveniently fits the hand of the user and may be provided with a seriesof pushbutton switches 37 for communicating with the computer 2 withwhich the digitizer 10 is used.

The digitizer tablet 40 is generally rectangular and has a planar topsurface 41 on which the cursor 20 moves. Positions on the top surface 41can be expressed as values in a two-dimensional coordinate system, andthe coordinate system used in the preferred embodiment is the orthagonalX-Y coordinate system. The digitizer tablet 40 comprises patternedelectrodes 50 and 60 which respectively couple with the cursor andprovide a signal indicative of the position of the cursor on thedigitizer tablet 40. With specific reference to FIG. 2, the digitizertablet comprises a layer A including the conductive electrodes of thedigitizer tablet as are more fully shown and explained in FIGS. 3 and 4.The digitizer tablet 40 includes a top layer 42 of dielectric material,such as polyethylene, ABS plastic, Mylar® or Teflon®, covers theelectrodes and provides a smooth upper surface of the digitizer tablet.Another layer of dielectric material 43 is provided under the electrodelayer A, and separates the electrode layer A from a conductive groundplane 44, which is not essential but is preferably provided as a shieldagainst external noise. A plastic bottom cover 45 completes thedigitizer tablet 40. The digitizer tablet 40 is connected to thedigitizer electronics by a cable 48, the ground shield of which isconnected to the ground plane 44 (not shown) and the other conductors ofwhich are connected as described below.

The electrode layer A of the digitizer tablet 40, a small portion ofwhich is seen in FIG. 1, is best illustrated in FIGS. 3 and 4. Thedigitizer tablet 40, and particularly electrode layer A thereof,comprises a patterned electrode 50 for the X-axis, a patterned electrode60 for the Y-axis, and an electrode 70 providing a balance signal. Theforegoing electrodes are laid out in a substantially rectangular overallconfiguration, and are preferably surrounded by a perimeter shieldelectrode 80.

The X-axis electrode 50 comprises a plurality of stripes, e.g. stripes51, 52, 53 and 54, which are connected together along one upper edge ofthe tablet 40 by a X-axis bus 58. The stripes extend downwardly from thebus 58 across most of the tablet. The stripes are parallel to each otherand have their center lines evenly spaced apart, but the individualstripes increase progressively in width from one side of the tablet tothe other. Thus, the stripe 52 is wider than the adjacent stripe 51, andstripe 53 is wider than adjacent stripe 52. Electrical signalaccumulated on the stripes is conducted via the bus 50 to an X-axissignal wire 59 comprising one conductor of the digitizer tablet cable48.

The Y-axis electrode 60 has a sawtooth pattern with individual sawtoothsinterleaved with the striped pattern of the X-axis electrode. Withcontinued reference to FIG. 3, the Y-axis electrode comprises aplurality of sawtooths 61 extending substantially across the digitizertablet 40, the sawtooths 61 being connected together at the edge of thetablet by a Y-axis bus 68. Each individual sawtooth 61 comprises aelongated, narrow area of the Y-axis electrode 60, having edges 62 and63 which converge as the sawtooth extends from the bus portion 68 to anarrow, terminal end 64 adjacent the X-axis bus conductor 58. Thesawtooths 61 are evenly spaced apart, and are spaced intermediateadjacent stripes. The Y-axis electrode 60 has its bus 68 connected tothe digitizer electronics 100 via conductor 69 which is a portion ofcable 48.

The electrode layer A of the digitizer tablet 40 further comprises abalance electrode 70, which covers the surface of the electrode layerwhich remains between the stripes and sawtooths of the X-axis and Y-axispatterned electrodes. Again with reference to FIG. 3, the balanceelectrode 70 takes the form of elongate legs, e.g. legs 71, 72 and 73,wherein leg 71 and 72 flank X-axis stripe 51, and legs 72 and 73 flankthe next adjacent sawtooth 61. The legs of the balance electrodes areconnected adjacent the X-axis bus 50 and the Y-axis bus 68, e.g. legs 71and 72 are connected at 74, between the end of stripe 51 and the Y-axisbus 68. The balance electrode has conductor 75, comprising a portion ofcable 48, for connecting it into the digitizer electronics 100.

The electrode layer A further comprises a shield electrode 80, which isdeployed about the perimeter of the X-axis, Y-axis and balanceelectrodes. The shield electrode 80 may be connected to the shield ofthe cable 48, as indicated in FIG. 3.

The digitizer tablet, of course, has a useful surface confined to thearea of the stripes and sawtooths, respectively, of the X-axis andY-axis electrodes, and the useful digitizing surface does not includethe X-axis and Y-axis buses nor the perimeter shield electrode.

There is separation between the electrodes, so that they are separatelyconducting. This space is best illustrated in FIG. 4, which is anenlarged fragmentary view of the electrode layer A shown in FIG. 3. Withmore particular reference to FIG. 4, there are shown sawtooths 61 of theY-axis electrode 60, stripes 51, 52 of the X-axis electrode 50, and legs71, 72 and 73 of the balance electrode 70. A small amount of spacing,indicated by the letter S, is provided between all of the adjacentelectrode edges.

It will be appreciated that even FIG. 3 is shown very much enlarged andschematically for purposes of illustration, and that the actual stripes,sawtooths, and their spacing are much more fine than are shown in theFigure. In a preferred embodiment, there may be approximately fiveX-axis pattern stripes and five Y-axis pattern sawtooths per inch. Theamount of increase in width from one X-axis stripe to the adjacentX-axis stripe, may be approximately 0.001 inches.

It will further be appreciated that the coupling electrode 30 of thecursor 20 is sized and spaced from the tablet such that its field coversa plurality of stripes and sawtooths, preferably at least five. Thus,several elements of the patterned electrodes are coupled to the cursorat any given time, so that the position of the cursor with respect toany individual element is not a factor in the resultant signal.

It should also be appreciated at this time that stylus of the typecomprising an electrode in the shape of a ring or other suitable shapesurrounding a pointer of pen-like shape may be used in place of thecursor, if desired, provided the stylus electrode is sufficiently spacedfrom the tablet electrodes to spread electrical field coupling betweenthe stylus and the tablet electrodes, thereby achieving an averagingeffect with respect to any particular stripe or sawtooth element.

The X-axis electrode 50, Y-axis electrode 60, balance electrode 70 andshield electrode 80 may comprise copper, silver, conductive ink or anyother conductor deposited on a printed circuit board material, whereinthe printed circuit board material comprises the dielectric layer 43 ofthe digitizer tablet 40. Other fabrication techniques are also useful,including printing the electrodes in conductive ink on a thin film ofMylar® or similar material, and affixing the film with the electrodesprinted thereon to either the dielectric layer 43 or the top dielectriclayer 42 of the digitizer tablet 40.

One advantage of the digitizer according to the invention herein is thatit does not depend upon accurate resistivity of the conductive elements,whereby a certain amount of variation in the thickness of the conductiveelectrodes does not produce error in the output of the digitizer. Thispermits less expensive fabricating techniques to be used in preparingthe digitizer tablet.

The digitizer 10 operates by electrostatic coupling between the couplingelectrode 30 of the cursor 20 and the patterned X-axis electrode 50, thepatterned Y-axis electrode 60 and the balance electrode 70 of thedigitizer tablet 40. In the preferred embodiment, an input signal isprovided to the cursor, coupled to the electrodes of the tablet, and theresultant signals on the tablet electrodes are processed to determine anX-axis coordinate and Y-axis coordinate of cursor position on thetablet. With reference to FIGS. 5 and 6, the eIectronic operation of thedigitizer 10 is illustrated. With reference first to FIG. 5, the cursor20 is shown positioned on the digitizer tablet 40 at the left hand sideof the figure. The cursor 20 and tablet 40 are shown schematically, andin particular the tablet would generally be less elongated and thecursor would be smaller with respect to the tablet. The right handportion of FIG. 5 generally comprises the electronics package 100 of thedigitizer 10.

The X coordinate is given by the signal coupled from the cursor to thevariable width stripe patterned X-axis electrode. The Y coordinate isgiven by the signal coupled by the cursor to the sawtooth patternedY-axis electrode. The balance electrode provides a third signal,referred to as the "B" signal below. The sum of the signals from theX-axis, Y-axis and balance electrodes gives the strength or magnitude ofthe total signal coupled from he cursor. The absolute X-axis and Y-axiscoordinates are given by:

    X coordinate proportional to X/X+Y+B;

    Y coordinate proportional to Y/X+Y+B;

where "X" equals the X-axis signal, "Y" equals the Y-axis signal and "B"equals the balance signal. This ratiometric measurement techniqueeliminates the error otherwise caused by variation in distance betweenthe cursor and the digitizing tablet.

Again with reference to FIG. 5, the electronics 100 for carrying out theratiometric measurement comprise a square wave oscillator 105 providinga square wave output signal 110 (FIG. 6) at a fixed frequency,preferably in the range of 10 KHZ to 1 MHZ. The square wave oscillator105 drives a tuned LC circuit comprised of inductor 106 and capacitor107, with the output signal of the square wave oscillator passingthrough a resistor 108 for limiting the loading on the oscillator. Thetuned circuit elements 106 and 107 provide a multiple voltage gain todrive the coupling electrode 30 through conductor 26 of cable 25. thecursor signal is shown at 115 in FIG. 6, and is phase shifted 90° fromthe oscillator output by operation of the LC circuit.

The signal on the coupling electrode 30 electrical field couples to theX-axis, Y-axis and balance electrodes of the digitizer tablet 40, withthe strength of the signal coupled to the X-axis and Y-axis electrodesbeing dependent upon the position of the cursor on the tablet 40. Moreparticularly, in accordance with the arrangement of the electrodesilllustrated, a greater Y-axis signal is provided when the cursor isnear the lower edge of the tablet, where the sawtooths of the Y-axispatterned electrode are wider. Similarly, the X-axis signal is largerfor cursor positions toward the right hand side of the tablet, where thestripes of the X-axis patterned electrode are wider.

The signals from the X-axis, Y-axis and balance electrodes are providedon conductors 59, 69 and 75, respectively, and these signals areprocessed as also illustrated in FIGS. 5 and 6. The X-axis signalprocessing is indicated generally at 120, and it will be appreciatedthat the Y-axis signal processing 140 and balance signal processing 145are similar and showing them would be merely repetitious.

The signal from the X-axis electrode, provided on conductor 59, isamplified by the current-to-voltage amplifier 122 to produce signal 125shown in FIG. 6. This signal is phase-shifted another 90° from theoscillator, because of the coupling. The amplified signal goes through ahigh pass filter comprised of capacitor 123 and resistors 126 and 127 toreject any 60 cycle line frequency noise. This filtered AC signal issynchronously switched at the frequency of the square wave oscillator105 by switch 130, the filtered AC signal being applied through switch130 is the input to an operational amplifier 131.

An important feature of the signal processing in the digitizer 10according to the invention herein is the synchronous processing of theX-axis, Y-axis and balance electrode signals. In the X-axis signalprocessing 120 shown, the X-axis signal is provided to the operationalamplifier 131 through switch 130 operated at the frequency of the squarewave oscillator 105, which is also generating the input signal to thecursor. This synchronous processing of the signals rejects noise orother pickup at all other frequencies, resulting in a very high signalto noise ratio and improved accuracy in the determination of thecoordinate position of the cursor on the digitizer tablet 40.

Capacitor 132 and resistor 133 act as a filter to reduce AC ripple onthe output of the operational amplifier 131, whereby a stable voltagesignal indicative of the X-axis position of the cursor is produced online 134 labeled "XDC". With reference to FIG. 6, the X-axis signal atpoint P, is shown at 136, and comprises a positive portion shown at 137which is pulled to virtual ground, as shown at 138 when switch 130 isclosed. Point Q, at the input of the operational amp 131 is alsomaintained at virtual ground by the feed back through capacitor 132 andresistor 133. The voltage gain is mainly determined by the ratio of thevalue of resistor 133 over the value of resistor 126, and the output ofoperational amplifier 131 is as noted above, a DC signal having a valueindicative of the X-axis position of the cursor on the tablet 40. Theoutput of the X-axis signal processing electronics 120 is shown at 135in FIG. 6.

The Y-axis electrode and balance electrode signals are processedsimilarly.

The outputs of the X-axis signal processing 120, the Y-axis signalprocessing 140 and the balance signal processing 145 are furtherproceeded in a microprocessor 150. More particularly, the signals areconverted from analog to digital values via an analog-to-digitalconverter 155, which operates on the signals sequentially by means ofswitches 151, 152 and 153 which are also operated by the microprocessor.The rate of sampling may coincide with the report rate of themicroprocessor to the computer 2, typically 10 to 120 HZ. Themicroprocessor calculates the position of the cursor in accordance withthe ratiometric measurement technique and formula set forth above, andprovides the position to the computer 2 for use and/or display.

Alternatively, the digitizer tablet can also be operated by sequentiallyinputting signal to the X-axis, Y-axis and balance electrodes, andutilizing the cursor electrode as the output electrode. The operation inthis mode is also carried out synchronously, both to achieve high signalto noise ratio and also to maintain the necessary distinction betweenthe three signals being handled.

Accordingly, there has been described an improved digitizer whichadmirably achieves the objects of the invention herein. It will beappreciated by those skilled in the art that various changes can be madefrom the preferred embodiments described above without departing fromthe spirit and scope of the invention, which is limited only by thefollowing claims.

I claim:
 1. A digitizer tablet having a pattern of conductive electrodeportions arranged in a plane, said pattern comprising a set of generallyrectangular conductive areas each including at least one planarelectrode, the longer sides of said generally rectangular conductiveareas being arranged parallel to a first coordinate axis and the shortersides of said generally rectangular surface areas having a length whichincreases incrementally in a direction parallel to a second coordinateaxis for successive generally rectangular surface areas, and a set ofgenerally triangular conductive areas each including at least one planarelectrode, said generally triangular conductive areas being interleavedbetween said generally rectangular conductive areas, the shortest sideof said generally triangular conductive areas being arranged parallel tosaid second coordinate axis and the remaining two sides of eachgenerally triangular conductive area being separated by a distance in adirection parallel to said second coordinate axis which increaseslinearly in a direction parallel to said first coordinate axis.
 2. Thedigitizer tablet as defined in claim 1, wherein one of the shorter sidesof each of said set of generally rectangular conductive areas iselectrically connected to a first conductive bus means, thereby forminga first electrode array, and the shortest side of each of said generallytriangular conductive areas is electrically connected to a secondconductive bus means, thereby forming a second electrode array.
 3. Thedigitizer tablet as defined in claim 2, wherein each of said generallytriangular conductive surfaces is truncated at the vertex opposing saidshortest side.
 4. The digitizer tablet as defined in claim 2, whereinsaid first and second electrode arrays are arranged on a rectilineararea in said plane, the portions of said rectilinear area not occupiedby said first and second electrode arrays being substantially covered bya third set of conductive areas electrically connected in a serpentinepattern, thereby forming a third electrode array, except that said firstand second electrode arrays are respectively electrically insulated fromsaid third electrode array by narrow strips of insulating materialconnected in respective first and second serpentine patterns.
 5. Adigitizer comprising:(a) a digitizer tablet having first couplingelectrode means arranged in a pattern in a plane; (b) second couplingelectrode means movable over said plane; (c) signal generating meanscoupled to one of said first and second coupling electrode means fordriving said one coupling electrode means; and (d) processing meanscoupled to the other of said first and second coupling electrode meansfor processing the signals electrostatically induced in said othercoupling electrode means in response to driving of said one couplingelectrode means, the processing means being adapted to determine thecoordinates of a point on said second coupling electrode means relativeto a reference system on said plane having first and second coordinateaxes; wherein said pattern comprises a set of generally rectangularconductive areas each including at least one planar electrode, thelonger sides of said generally rectangular conductive areas beingarranged parallel to a first coordinate axis and the shorter sides ofsaid generally rectangular surface areas having a length which increasesincrementally in a direction parallel to a second coordinate axis forsuccessive generally rectangular surface areas, and a set of generallytriangular conductive areas each including at least one planarelectrode, said generally triangular conductive areas being interleavedbetween said generally rectangular conductive areas, the shortest sideof said generally triangular conductive areas being arranged parallel tosaid second coordinate axis and the remaining two sides of eachgenerally triangular conductive area being separated by a distance in adirection parallel to said second coordinate axis which increaseslinearly in a direction parallel to said first coordinate axis.
 6. Thedigitizer as defined in claim 5, wherein one of the shorter sides ofeach of said set of generally rectangular conductive areas iselectrically connected to a first conductive bus means, thereby forminga first electrode array, and the shortest side of each of said generallytriangular conductive areas is electrically connected to a secondconductive bus means, thereby forming a second electrode array.
 7. Thedigitizer as defined in claim 6, wherein each of said generallytriangular conductive surfaces is truncated at the vertex opposing saidshortest side.
 8. The digitizer as defined in claim 6, wherein saidfirst and second electrode arrays are arranged on a rectilinear area insaid plane, the portions of said rectilinear area not occupied by saidfirst and second electrode arrays being substantially covered by a thirdset of conductive areas electrically connected in a serpentine pattern,thereby forming a third electrode array, except that said first andsecond electrode arrays are respectively electrically insulated fromsaid third electrode array by narrow strips of insulating materialconnected in respective first and second serpentine patterns.
 9. Adigitizer comprising:(a) a digitizer tablet having first couplingelectrode means arranged in a pattern in a plane; (b) second couplingelectrode means movable over said plane; (c) signal generating meanscoupled to one of said first and second coupling electrode means fordriving said one coupling electrode means; and (d) processing meanscoupled to the other of said first and second coupling electrode meansfor processing the signals electrostatically induced in said othercoupling electrode means in response to driving of said one couplingelectrode means, the processing means being adapted to determine thecoordinates of a point on said second coupling electrode means relativeto a reference system on said plane having first and second coordinateaxes; wherein said pattern is defined in part by first and secondpluralities of straight edge lines, said straight edge lines of saidfirst plurality being parallel to said first coordinate axis and saidstraight edge lines of said second plurality being oblique to said firstcoordinate axis, said first coordinate being determined by saidprocessing means by detecting an induced signal corresponding to theamount of electrostatic coupling between said first coupling electrodemeans and a portion of said second coupling electrode means formed by aplurality of conductive areas each defined in party by a pair ofstraight edge lines, at least one of which is of said second plurality,the distance in a direction parallel to said second coordinate axisbetween said straight edge lines of said pair increasing linearly in adirection parallel to said first coordinate axis, and said secondcoordinate being determined by said processing means by detecting aninduced signal corresponding to the amount of electrostatic couplingbetween said first coupling electrode means and a portion of said secondcoupling electrode means formed by a plurality of conductive areas eachlying between a pair of straight edge lines of said first plurality, thedistance in said direction parallel to said second coordinate axisbetween said straight edge lines of successive pairs of straight edgesof said first plurality increasing incrementally in said directionparallel to said second coordinate axis.
 10. A digitizer tabletcomprising a plurality of conductive elements deployed in a commonplane, wherein:(a) the areas of first selected ones of said conductiveelements varying as a function of position in a first coordinate of saidcommon plane; (b) the areas of second selected ones of said conductiveelements varying as a function of position in a second coordinate ofsaid common plane; and (c) the combined areas of selected ones of theconductive elements are independent of position on the common plane. 11.A digitizer tablet as defined in claim 10 wherein the first and secondcoordinates are the X - Y coordinates of the Cartesian coordinatesystem.
 12. A digitizer tablet as defined in claim 11 wherein the firstselected conductive elements are triangular elements providing acontinuous change in area in the first coordinate direction and thesecond selected coordinates are elongated rectangular elements ofincrementally varying dimension in the direction of the secondcoordinate, wherein position representative area encompasses more thanone conductive element providing a substantially continuous change inarea in the second coordinate direction.
 13. A digitizer tablet asdefined in claim 12 wherein the conductive elements further includethird conductive elements which are selectable to provide combined areasindependent of position on the common plane.
 14. A digitizer tablet asdefined in claim 13 wherein the third conductive elements coversubstantially the remaining planar surface area, except fornon-conductive gaps between the elements, to complete the combined areasof conductive elements which provide a reference area independent ofposition.
 15. A digitizer tablet as defined in claim 10 and furthercomprising a coupling element movable over the digitizer tablet, whereinthe coupling element is adapted to couple with a localized area of thedigitizer tablet including a sufficient portion of the first and secondselected ones of the conductive elements to ascertain position of thecoupling element.
 16. A digitizer comprising:(a) a digitizer tablethaving a first electrode having a pattern that varies with a firstcoordinate in an orthogonal X - Y coordinate system including aplurality of conductive stripes connected together along one end, thestripes having progressively increasing width starting from a side ofthe tablet and extending toward an opposite side of the tablet, and asecond coplanar electrode having a pattern that varies with the secondcoordinate in the orthogonal X - Y coordinate system including aplurality of elongated sawtooths interleaved between the stripes, thesawtooths also being connected together at one end thereof: (b) acoupling electrode movable on the digitizer tablet for coupling with thefirst and second patterned electrodes of the digitizer tablet; (c)signal generating means connected for inputting an electrical signalalternately to the first and second patterned electrodes of thedigitizer tablet, thereby establishing electrostatic field coupling withan electrical signal on the coupling electrode, the signal on thecoupling electrode being alternately indicative of the position of thecoupling electrode in each coordinate of the orthogonal X - Y coordinatesystem; and (d) electronic means for calculating the coordinates of thecoupling electrode in the orthogonal X - Y coordinate system from thesignals coupled to the coupling electrode from the first and secondpatterned electrodes.
 17. A digitizer as defined in claim 16 and furthercomprising a balance electrode coplanar with the stripe and sawtoothpatterned electrodes and occupying the space therebetween, wherebyapplying the input signal to the stripe, sawtooth and balance electrodesprovides a reference signal on the coupling electrode and the X-axiscoordinate of the coupling electrode is proportional to the X-axissignal divided by the reference signal, and the Y-axis coordinateposition is proportional to the Y-axis signal divided by the referencesignal.
 18. A digitizer as defined in claim 17 wherein the field of thecoupling electrode is circular and covers at least four stripes andsawtooths.
 19. A digitizer as defined in claim 18 wherein the couplingelectrode is mounted in a cursor having a central opening transparent tothe tablet surface, in which a mark indicates the center of the couplingelectrode.
 20. A digitizer as defined in claim 17 wherein the digitizertablet further comprises a perimeter shield electrode coplanar with andsurrounding the stripe, sawtooth and balance electrodes, for shieldingthem against interference.
 21. A digitizer as defined in claim 17 andfurther comprising a shield electrode positioned on the opposite side ofthe coupling electrode from the digitizer tablet and separated from thecoupling electrode by a dielectric spacer, for shielding the couplingelectrode from interference.
 22. A digitizer as defined in claim 21wherein the shield electrode is larger than and overhangs the couplingelectrode.
 23. A digitizer as defined in claim 20 wherein the digitizertablet further comprises a planar ground electrode positioned on theopposite side of the stripe, sawtooth and balance electrodes from thecoupling electrode and separated from the stripe, sawtooth and balanceelectrodes by a dielectric layer of the digitizer tablet.
 24. Adigitizer as defined in claim 17 wherein the stripes of the stripepatterned electrode are evenly spaced apart and progressive increase ofthe width of the stripe electrodes is approximately 0.001 inch, and thesawtooths of the sawtooth patterned electrode ar evenly spaced apart.25. A digitizer as defined in claim 24 wherein there are at least foureach striped and sawtooth electrodes per inch.
 26. A digitizer asdefined in claim 17 wherein the digitizer tablet comprises first andsecond layers of dielectric material with the first and second patternedelectrodes and the balance electrode therebetween.
 27. A digitizer asdefined in claim 26 wherein the first and second patterned electrodesand the balance electrode comprise a conductive material deposited onone of the first or second layers of dielectric material.
 28. Adigitizer as defined in claim 26 wherein the first and second patternedelectrodes and the balance electrode comprise a conductive ink printedon a thin film of dielectric material and said film is deployed betweenthe first and second layers of dielectric material.
 29. A digitizer asdefined in claim 26 and further comprising a perimeter shield electrodesurrounding the first and second patterned electrodes and the balanceelectrode, the perimeter shield electrode being positioned between thefirst an second layers of dielectric material.
 30. A digitizer asdefined in claim 26 wherein the first layer of dielectric comprises thetop surface of the digitizer tablet on which the coupling electrode isused, and further comprising a ground plane electrode substantiallycovering the second dielectric on the opposite side from the first andsecond patterned electrodes and the balance electrode.
 31. A digitizeras defined in claim 17 wherein the input to the digitizer electrodes isan alternating current electrical signal, and the electrical outputsignals from the coupling electrode is also alternating signals at thesame frequency, wherein the output signals from the coupling electrodeis detected synchronously with the input signal, thereby minimizinginterference and providing a high signal-to-noise ratio in the output ofthe digitizer tablet.
 32. A digitizer as defined in claim 31 wherein thefrequency of the input signal is in the range of 10 KHZ to 1 MHZ.